Polyesters are utilized in many applications wherein it is important for them to have high strength and good thermal properties. For instance, polyesters are commonly utilized in making tire cords and molded articles wherein high tensile strength is required. Such tire cords are commonly made out of polyethylene terephthalate which has respectable tensile strength. However, it is highly desirable to modify the polyethylene terephthalate in a manner that increases its tensile strength. For example, U.S. Pat. No. 4,605,728 discloses that the mechanical and thermal properties of polyesters can be improved by incorporating bis-hydroxyalkyl pyromellitic diimides therein.